3/4" is almost exactly 19 millimeters. 19,05 to be precise. The most common spark plug dimensions are 13/16 and 11/16". And for chainsaws and other small engines 3/4". Almost all Honda cars after 1983 have 11/16, those before that have 13/16".:readit:My GL1000 took a 3/4 deep socket. i don't know if that would be an 18mm or ???? :?
close but no cigar. 19mm and 3/4 are usually considered interchangeable. My Snap-on 3/4 has stamped on it " 3/4 19mm"you said not to laugh tooooooo hard
btw 3/4 and 18mm are extremely close in size